A. Field
This invention generally relates to methods and compositions for improving the strength and longevity of secondary roadways through environmentally sound practices.
B. Description of the Related Art
The engineering and construction of secondary roads (hereafter, “gravel roads,” “earth roads,” or “unpaved roads”) has been perpetually plagued by two interrelated problems: the deterioration of the road due to water, and the loss of surface cohesion and road compaction due to traffic. The deleterious effects of water on roadways, in particular, are well documented in the prior art. In cold weather, moisture that penetrates a road's base layers freezes and rips cracks into the road substrate that seriously undermine the load bearing capacity and longevity of the roadway. Likewise, in milder weather, when water seeps into the road's base layers it results in softening and erosion that causes potholes that are an expensive and recurring problem. And if the potholes are not immediately repaired, they fill with water and further exacerbate the deterioration of the roadway.
The impact of water on secondary roads—such as rural roads, access roads, field and forestry roads, or mountain roads—is especially pronounced because the quality of the surfacing materials is lower than in an asphalt paved road, for example, and thus provides reduced surface protection from the elements. Additionally, because of capillary action, water also seeps into the road base from the sides and bottom of the road's base or sub-base. Compared to sealed or “paved” roads, which require large machinery to pour concrete or to lay and smooth a bitumen-based surface, secondary unpaved roads are relatively easy and inexpensive to build. But unpaved roads require much more frequent maintenance—particularly after wet periods or when faced with increased traffic—and are generally prone to other problems not associated with paved roads.
For example, many secondary roads of either an earth or gravel variety—utilize native soils, often in conjunction with gravel quarried from local resources, to create the road's sub-base and base layers. Unfortunately, native soils and gravel are not always of suitable quality, resulting in a road base with diminished physical and mechanical properties. When secondary roads are constructed of poor road base materials, routine maintenance is not strictly employed, and the road is exposed to heavy moisture and/or traffic, the erosion of the road—due to damage to the road surface, sub-base, and base materials—is hastened.
Defects in road surfaces are typically classified into two categories: surface deterioration and surface deformation. While surface deterioration is related mostly to the quality of the surfacing materials and the way they respond to weather or traffic stresses, surface deformations often have combined causes that include both stresses to the road surface itself and other factors such as sub-base and base capacity and stability. Surface deterioration is exemplified by “dust,” the result of loss of fine binder material from road surfaces. Dust is a substantial problem for secondary roads, as the loss of these fine materials leads to other types of road distress such as loss of cohesion and compaction of the road fill material, and reduced capacity to maintain the requisite moisture in the road fill.
Surface deformations include ruts, corrugations, depressions, and potholes. Ruts are longitudinal depressions in the wheel paths caused by high moisture content, inadequate strength in the subsurface soil or base, inadequate surface course thickness, or heavy traffic loads. Corrugating or “washboarding” is a series of ridges and depressions across the road surface caused by lack of surface cohesion. Depressions are localized low areas one or more inches below the surrounding road surfaces that are caused by settlement, excessive moisture content, and/or improper drainage. Potholes are small depressions or voids in the road surface one or more inches deep which are caused by excessive moisture content, poor drainage, weak sub-base or base, poorly graded aggregate, or a combination of these factors.
As such, the problems typically associated with secondary roads—both surface deterioration and deformation—are caused by: 1) the harmful effects of water and high moisture content, including settlement and erosion, on the road surface and base, 2) the lack of surface cohesion and resulting loss of road compaction caused by dust, and 3) the heavy traffic loads exerted on roads with weak or inadequate soil, sub-base, or base. Industry has provided for the addition of various chemical additives to impart water repellency on road materials, with varying degrees of success and environmental impact. However, water repellant chemicals are not binders, and load bearing capacity, stability, and frost resistance are not improved by their application to the soil or road base. In many cases, dust can also be reduced on gravel roads by applying chemical additives commonly known in the art as “dust suppressors” or “dust retardants”) which draw moisture from the air to improve fine aggregate cohesion. And “soil stabilizers,” which are chemicals designed to act as binders and coalesce forming bonds between the soil or aggregate particles, have shown promise in greatly improving the load bearing and traffic capacity of the road. But existing soil stabilizers and dust retardants are difficult to apply and use in cold climates, tend to have long cure times, short life-cycles, and do not provide the requisite protection against water damage; particularly excessive moisture content resulting from capillary action.
Repairing damaged roadways by conventional methods can be extremely expensive, time consuming, and environmentally disruptive because the entire compacted gravel layer of the road must be replaced. Excavating the roadbed of a 1-km portion of road measuring 4 m in width produces about 2000 cubic meters (m3) of earthy waste; in a conventional road bed repair project, this would require roughly 220 truckloads of waste to be removed from the worksite, with 220 truckloads of new gravel being shipped back the worksite to complete the project. In isolated locations, or locations with difficult terrain, the expense of removing and later replacing the gravel is exorbitant as is the impact on local residents (who must cope with noise and air pollution), normal users of the roadway (who experience detours or extended delays during repair), and the landfills that store the removed waste. Conventional binders are a liquid asphalt, which turns into a black heat-absorbing road surface. At installation a conventional chip sealed surface must be swept resulting in the loss of as much as 20% of the chips installed.
As a result, there is a need in the art for an improved method of building up roads to create strength and longevity, wherein road builders will be able improve the longevity of the roadway, impart increased load bearing and traffic capacity, and reduce the time, costs, and environmental impact associated with conventional road repair projects.